Update CONTRIBUTE.md

adding release checklist
adding support for cssproc
adding docs
adding docs
review feedback
update yui to 3.11.0
add history entry of yui version update
adding unit tests for the new cssproc feature. organizing the tests better
bumping version 0.5.6
update yui version
Merge pull request #19 from lzhan/updateyui

update yui to 3.11.0
update yui version for example
Merge pull request #20 from lzhan/updateyui

update yui version for example
adding css-modules example
better error control in examples
merging upstream
typo
Merge pull request #18 from caridy/cssproc

adding support for `app.yui.plugin({ cssproc: true })`